This invention relates to a tetraazaporphin, a process for producing the same, an optical recording medium using the same, and a process for producing the optical recording medium.
Recently, application of a semiconductor laser light is proposed for writing or reading out in compact discs, video discs, liquid crystal display devices, optical reading machines, etc. or as a light source for electrophotography. Particularly, in the case of optical recording media using semiconductor laser, since a recording or reading head does not contact with a recording medium, the recording medium is characterized by no-friction. Thus, development and research of various recording media have been made. Particularly, in the field of a heat mode recording method, low melting point metals, organic polymers or dyestuffs have been proposed as substances for melting, vaporizing or subliming. Organic thin films containing organic polymers or dyestuffs are known to have a low thermal conductivity, or a low melting or subliming temperature. Thus, various substances such as cyanine dyestuffs, squalium dyestuffs, which are preferable in recording sensitivity, have been proposed as materials for forming recording layers. For example, Japanese Patent Unexamined Publication No. 56-16948 proposes an optical recording medium using dyestuffs as a recording layer.
When cyanine dyestuffs which have properly high reflectance by themselves are used as a recording layer and a dyestuff thin film recording layer is used in a reflection type optical recording medium, the formation of a metal reflecting film is not necessary, the structure of the medium is simplified and deterioration of recording and reading properties can be prevented (e.g. Japanese Patent Unexamined Publication No. 60-7878). But the cyanine dyestuffs and other known dyestuffs have generally low stability against light, so that the dyestuffs are faded by repeated irradiation of light at the time of readout after writing information, and the carrier-to-noise ratio (C/N ratio) of readout is lowered. That is, there is a problem in that stability in reading is worse.
In order to overcome such a problem, it is proposed to use naphthalocyanine dyestuffs excellent in light-fastness as the recording layer (e.g. U.S. Pat. No. 4,725,525).
On the other hand, recording layers of information recording media are formed by a vacuum forming method such as a vacuum deposition method, a sputtering method etc., and a wet method such as a coating method, a dipping method, etc. The wet method is more advantageous than the vacuum forming method economically. Therefore, solubility of dyestuffs in organic solvents becomes important economically and technically in the course of forming recording layers.
Furthermore, in order to make information recording and reading devices smaller, a semiconductor laser is used. Recently, in order to improve a recording density, the emission wavelength is gradually shortened from about 800 nm to about 600 nm. In the case of using a semiconductor laser having such a shortened wavelength, there is a problem in that it is impossible to use known naphthalocyanine derivatives having a maximum absorption at about 810 nm and small absorptions in the wavelength region of 800 nm or less, and a small reflectance.